1. Field of the Invention
The invention relates to a method for transmitting data in a motor vehicle from an application using an Ethernet transport protocol. In particular, the data are time-critical streaming data that have particular QoS (Quality of Service) requirements that are also prescribed, in particular by a fixed transmission frequency, defined according to standard, and a maximum permitted transmission time, which can be determined by the application. A typical instance of application for such data is audio and/or video data, or else control data that are transmitted in a form encapsulated into data packets within the context of an Ethernet protocol.
2. Related Art
The transmission of the data between nodes of a motor vehicle, which may be particularly (at least to some extent also) in the form of controllers of the motor vehicle, for example in the form of multimedia controllers, takes place particularly on the basis of the Ethernet AVB transport protocol provided for such streaming data. The encapsulated content (data) is in this case transmitted preferably together with supplementary information, for example about the sampling rate, the number of audio channels, the video format and the resolution thereof and/or the compression of the data. This is implemented by the IEEE 1722 standard.
When data are transmitted by this Ethernet transport protocol, provision is made for the data to be transmitted via an Ethernet-based network at cyclic intervals. The method also provides for the local transmitters and receivers (PHY) of a node, usually in the form of transceivers, to be deactivated in non-use periods, in which no data need to be transmitted, and to be activated again when data are pending transmission, wherein the local transmitters and receivers (PHY) are transferred from an operating mode (active) to a quiescent mode (LPI) in a deactivation time (Ts) and are transferred from the quiescent mode to an operating mode in an activation time (Tw).
The minimum cycle time (TCT) is thus obtained from the sum of the active transmission time for the data, also called frame transmission time (TFRM), and the changeover times, i.e., the deactivation time (TS) and the activation time (TW). In order to be able to save power, the cycle time must thus be longer than the minimum cycle time (TCT) so that the local transmitters and receivers are in the quiescent mode for a certain period. The local transmitters and receivers of a node, which are also called PHY devices and accomplish particularly the bit-by-bit data transmission in the physical layer, are thus deactivated in non-use periods, in which no data need to be transmitted, and activated again when data are pending transmission in a transmission frame (MAC frame) of an upstream layer, particularly what is known as the MAC layer.
Besides the typical bus systems in the automotive field, such as CAN bus, FlexRay or the like, a bus system operating on the basis of an Ethernet transport protocol is also increasingly finding its way into the motor vehicle. The Ethernet, i.e., a network that operates on the basis of the Ethernet transport protocol, and is usually wired, has a high bandwidth, is highly flexible and has worldwide standardization. Therefore, the Ethernet will also be an important system interface for a motor vehicle in the coming years.
The increasing electrification of motor vehicles also means an increasing rise in the power consumption thereof. This in turn results in increased fuel consumption, which also has a direct effect on the end user in terms of cost. In addition, taxation on a motor vehicle is today calculated on the basis of CO2 (carbon dioxide) emissions, which can in turn be derived from the energy consumption in terms of fuel. The range of an electrically operated vehicle is also coupled to the capacity of the battery and hence to the power requirement of the loads connected in the motor vehicle.
During standard network operation, the local transmitters and receivers (PHY devices), which are also called Ethernet transceivers, have a constant power requirement that is independent of the utilization level of the connection in the data transmission, since what are known as IDLE code groups are sent when no useful data need to be transmitted via the data connection. This power consumption exists during the changeover phases for activating and/or deactivating the PHY devices.
A new IEEE 802.3az standard (also called Energy Efficient Ethernet—EEE) provides the previously described expansions in order to deactivate the transmission of IDLE code groups in the local transmitters and the local receivers on the other side of the communication connections during the periods without useful data transmission instead of continuing to send the IDLE code groups. This deactivation is also called Low Power Idle—LPI (energy saving mode or quiescent mode). This allows the power requirement to be reduced in the physical layer, which physically produces the actual data transmission.
The aforementioned standard also stipulates the minimum transmission time between the normal state of the Ethernet transceiver, in which data transmission can take place, and the deactivated mode (LPI). In this case, the time for waking or activating a transmitter and/or receiver from the energy saving mode (LPI) is specified at TW=30 μs. In addition, a changeover time is stipulated that is needed in order to transfer the local transmitter and/or receiver to a quiescent mode (LPI state). This deactivation time Ts is Ts=200 μs according to the provided standard. The activation time Tw and the deactivation time Ts are the minimum values according to the standard and cannot be reduced, in order to remain compliant with the standard. Compliance with the standard is necessary in order to achieve a universal communication capability among the devices.
In order to save energy from an Ethernet AVB connection (Ethernet Audio Video Bridging), US 2011/0090914 A1 proposes a method in which an energy-efficient network (EEN—Energy Efficient Networking) is negotiated. In this case, the MAC controllers and the PHY transceivers negotiate a data rate for the connection, with a lower data rate reducing the power consumed by the transceivers. In order to maintain the connection and to avoid complex tuning of the PHY transceivers among one another (training), time windows of the Ethernet AVB connection are regularly used in order to update configuration parameters and/or training information. The disadvantage in this case, however, is that the data rate needs to be known beforehand in order to afford an appropriate setting option.
EP 2 073 464 A1 discloses a method in which the PHY transceivers transmit data on different data channels. When the data packet traffic is relinquished, some channels can be shut down or reset to an idle mode with relatively low energy consumption, the proposal being made that one or more of the quiet channels be used for transmitting control signals.
A further aspect of energy saving is described by US 2009/0158377 A1, which describes data transmission on the basis of the Ethernet AVB transfer protocol, the Ethernet cable connection being used to achieve not only the data transmission but also a supply of energy to the reception devices that process the received data further. Since the energy requirement of the reception devices for the further processing of the data is also dependent on the volume of the received data and hence on the transmission frequency of the data packets, inter alia, it is proposed that the energy provided via the Ethernet cable be made dependent on the transmission frequency, for example. The energy requirement for the actual communication engineering is not reduced thereby, however.
Before the method proposed in accordance with the invention is described in more detail, the general mechanism of data transmission on the basis of an Ethernet transport protocol will be explained briefly for the purposes of comprehension.
In an Ethernet network, the transceivers (Ethernet transceivers, PHY devices) in a first protocol layer (PHY layer), also called physical layer, allow the actual communication between connected network subscribers by physically sending and receiving the data packets. The connection control is performed in a second protocol layer (MAC layer, media access control layer, which is upstream of the first protocol layer), also called data link layer, by MAC controllers. The MAC controllers of the second protocol layer form transmission frames (MAC frames), in which the actual data are then compiled on a bit-by-bit basis and transmitted to the actual data transmission to the PHY layer. Data transmission takes place only when a transmission frame (MAC frame) in the second, protocol layer is pending transmission. In order to maintain the data connection, IDLE packets or IDLE code groups are sent when there are no data pending transmission. The actual applications, for example in controllers, are then found in protocol layers further upstream of the second protocol layer.
The Ethernet AVB transport protocol IEEE 1722 is increasingly becoming of interest in use in motor vehicles. This protocol sends data via an Ethernet-based network at cyclic intervals. Before the actual sending of the data, the required resources, for example the bandwidth and/or the transmission rate from the local transmitter to the local receiver, are reserved. In particular, this can also be performed with the dedicated MSRP protocol (Multiple Stream Reservation Protocol), which is part of the AVB standard 802.1Qat. This propagates the transmission cycle, inter alia. Typical transmission rates are 125 μs or 250 μs, which is significantly shorter than the minimum cycle comprising activation time Tw and deactivation time Ts. Within the context of this AVB standard 802.1Qat, energy saving by the energy efficient Ethernet (EEE), in the transmission direction of the full-duplex Ethernet connection, is therefore not possible.
Traffic shaping is a further function of the Ethernet AVB standard implemented by the Q802.3Qav standard. Traffic shaping affords the opportunity to control the flow of data from a node of the network, with a particular transmission rate and/or bandwidth being set. The basic idea of traffic shaping is to delay data packets arriving too quickly from the upper protocol layers in order to initiate uniform transmission to the physical transmission devices of the physical layer. This reservation message and the parameters contained therein set and adjust the traffic shaper of the respective output ports.
Since the typical transmission rate is higher than the minimum transmission cycle comprising activation time Tw and deactivation time Ts in this mode too, however, it is not possible to save any power or any energy in this mode. Instead, the data packets are merely delayed. The realtime response of the Ethernet AVB standard is thus adversely affected by energy efficient Ethernet (EEE).
The underlying problem is the activation time Tw that is always needed when leaving the deactivated state of the local transmitters and/or receivers in order to activate the local transmitters and receivers. According to the proposed standard, a local transmitter and/or receiver leaves the deactivated state only when a transmission frame (MAC frame) in which data are intended to be transmitted is available. Since the upper (upstream) layers of the data transmission protocol (communication model) are largely decoupled from the physical data transmission, it is thus always possible for a delay in the activation time Tw in the order of 30 μs to arise, for example in order to activate the local transmitter. It is then necessary for a transmission frame to wait, and said transmission frame is delayed by this time.